█ AGNES GALAMBOSI
Nuclear winter is a meteorological theory estimating the global climatic consequences of a nuclear war—or a natural disaster such as a major asteroid impact—that injects large amounts or dust or water vapor into the atmosphere. Nuclear winter models predict prolonged and worldwide cooling and darkening caused by the blockage of sunlight.
During the Cold War, concern about the use of nuclear weapons initially concentrated on initial blast damage and the dangers of radioactive fallout. Subsequently, researchers began to explore the possible environmental effects of nuclear war. The term nuclear winter was first defined and used by American astronomer Carl Sagan (1934–1996) and his group of colleagues in their 1983 article (later referred to as the TTAPS-article, from the initials of the authors' family names). This article was the first one to take into consideration not only the direct damage, but also the indirect effects of a nuclear war.
During a nuclear war, the exploding nuclear warheads would create huge fires, resulting in smoke and soot from burning cities and forests being emitted into the troposphere in vast amounts. According to nuclear winter theory, this would block the Sun's incoming radiation from reaching the surface of Earth, causing cooling of the surface temperatures. The smoke and soot soon would rise to high altitude because of their high temperature and drift there for weeks without being washed out. Finally, the particles would settle in the Northern Hemisphere mid-latitudes as a black particle cloud belt, blocking sunshine for several weeks.
The ensuing darkness and cold, combined with nuclear fallout radiation, would kill most of Earth's vegetation and animal life, which would lead to starvation and diseases for the human population surviving the nuclear war itself. At the same time, because the smoke would absorb sunlight, the upper troposphere temperatures would rise and create a temperature inversion causing further retention of smog at the lower levels. Another predicted consequence is that nuclear explosions would produce nitrogen oxides that would damage the protective ozone layer in the stratosphere and allow more ultraviolet radiation to reach Earth's surface.
Although the basic findings of the original TTAPS-article have been confirmed by later reports and sophisticated computer modeling, some later studies report a lesser degree of cooling that would last for weeks instead of the initially estimated months. In the extreme, however, depending on the number of nuclear explosions, their spatial distribution, targets, and many other factors, a cloud of soot and dust could remain for many months, reducing sunlight almost entirely and decreasing average temperatures to well below freezing over a majority of the densely inhabited areas of the Northern Hemisphere.
The nuclear winter scenario remains scientifically controversial because the exact level of atmospheric damage, along with the extent and duration of subsequent processes cannot be agreed upon with full confidence. Opponents of the nuclear winter theory argue that there are many problems with the hypothesized scenarios either because of the model's incorrect assumptions (e.g., the results would be right only if exactly the assumed amount of dust would enter the atmosphere, or because the model assumes uniformly distributed, constantly injected particles). Other critics of the nuclear winter scenario point out that the models used often do not include processes and/or feedback mechanisms that may moderate or mitigate the initial effects of nuclear blasts on the atmosphere (e.g., the moderating effects of the oceans). In contrast to nuclear winter models, some climate models actually postulate a "nuclear summer," resulting from a worldwide warming caused by many small contributions to the greenhouse effect from carbon dioxide, water vapor, ozone, and various aerosols entering the troposphere and stratosphere.
What all scenarios and models forecast, however, is that a nuclear war would have a significant effect on the atmosphere and climate of Earth. This in turn would drastically and negatively affect many aspects of life such as food production and energy consumption.
█ FURTHER READING:
International Seminar on Nuclear War and Planetary Emergencies, 20th Session: The Role of Science in the Third Millennium, Man-Made & Natural Disasters, Post-Berlin-Wall Problems-Nuclear Proliferation in the Multipolar World. Singapore: World Scientific Publishing, 1997.
Weinberger, Casper. "The Potential Effects of Nuclear War on the Climate." Nuclear Winter, Joint Hearing before the Committee on Science and Technology and the Committee on Interior and Insular Affairs, U.S. House of Representatives. Washington, D.C.: Government Printing Office, 1985.
Ehrlich, Paul, et al., "Long-Term Biological Consequences of Nuclear War." Science 222, 4630 (1983).
Turco, R. P., O. B. Toon, T. P. Ackerman, J. B. Pollack, and Carl Sagan. "Nuclear Winter: Global Consequences of Multiple Nuclear Explosions." Science 222, 4630 (1983).
White Paper. "Nuclear Winter: Scientists in the Political Arena." Physics in Perspective 3:1 (2001):76–105.